1. Field of the Invention
The present invention relates to an image forming method that forms an image by dry-processing an image forming medium containing a silver halide.
2. Related Background Art
Energies used to form or record an image include light, sound, electricity, magnetism, heat, radiations such as electron rays and X-rays, and chemical energy, among which, in particular, widely used are light, electricity, heat energy, or a combination of any of these.
For example, the image forming method that employs the combination of light energy with chemical energy includes a silver salt photographic process and a method in which a diazo copying paper is used. The method that employs the combination of light energy with electric energy includes an electrophotographic system. The method that utilizes heat energy also includes a method in which a thermal recording paper or transfer recording paper is used. On the other hand, there is a method that utilizes electric energy in which an electrostatic recording paper, electrothermal recording paper, or electrosensitive recording paper is used.
Of the above image forming methods, the silver salt photographic process can obtain an image having a high resolution. The silver salt photographic process, however, requires the developing and fixing that uses complicated liquid compositions, or the drying of an image (or a print).
Now, development is energetically made on image forming methods that can form an image through a simple processing.
For example, U.S. Pat. No. 4,629,676 teaches a method in which polymerization reaction under dry (thermal) conditions is caused by the photosensitive reaction of silver halide that acts as a trigger, to form an image comprising a polymer.
This method has the advantage that no complicated wet processing is not required, but has had the disadvantage that the polymer formation rate (i.e., polymerization rate of a polymeric compound) is so low that it takes a long time to form the polymer image. Incidentally, this disadvantage arises presumably because of a reaction intermediate (which functions as a polymerization initiator) formed in the course of heating, by the reaction between silver produced from silver halide by imagewise exposure and a reducing agent, which intermediate is so stable and has so low activity as the polymerization initiator that the polymerization reaction can not proceed so rapidly.
On the other hand, to cope with this problem to accelerate the polymerization, Japanese Patent Application Laid-open No. 52-70836 discloses a method in which a thermal polymerization initiator is used.
This method comprises forming a latent image comprising silver metal produced from silver halide by imagewise exposure, converting, under heating, a reducing agent into an oxidized product having a polymerization inhibitory powder different from that of said reducing agent by utilizing a catalytic action of the above silver metal, thereby producing a difference in the polymerization inhibitory power between the reducing agent and the resulting oxidized product and also causing a thermal polymerization reaction utilizing the thermal polymerization initiator, thus forming a polymer image corresponding to the difference in the polymerization inhibitory power.
This method, however, has the disadvantage that a good contrast can only be made with difficulty in the polymer image.
This disadvantage arises presumably because the oxidation-reduction reaction taking place in a latent image portion to form the oxidized product and the polymerization reaction to form the polymer image are allowed to take place in the same heating step, so that these reactions may proceed in a competitive fashion and thus the respective reactions may not proceed in a good efficiency.
The image formation according to this method is very unstable in that, for example, the areas on which the polymer is formed may turn into exposed areas or unexposed areas only because of a slight change in the amount of the reducing agent.
U.S. Pat. No. 4,649,098 also discloses a method in which a reducing agent having a polymerization inhibitory power is formed into an oxidized product by imagewise consumption (at imagewise exposed areas) in the course of the developing of silver halide, and, after imagewise inhibition (at imagewise unexposed areas) of polymerization reaction by the action of the residual reducing agent, light energy is uniformly applied (whole areal exposure) from the outside to cause photopolymerization at the area at which the reducing agent has been consumed (imagewise exposed area), thus forming a polymer image.
Our research group has proposed a method in which an image comprised of a coloring matter is formed on an image receiving paper by utilizing a difference in the rate of evaporation of coloring matters between a polymerized area and an unpolymerized area (U.S. Ser. No. 564,060 filed on Aug. 8, 1990 corresponding to Japanese Patent Application No. 1-205626).
When a multi-color image is formed utilizing the difference in the rate of evaporation of coloring matters between a polymerized area and an unpolymerized area, plural images with different colors must be formed superposingly on the same sheet of image receiving paper. It, however, has been technically very difficult to superpose plural images with different colors in a doubling-free (or misregistration-free) state.
To overcome the above problem, our research group has proposed an image forming method that can superpose images in a doubling-free state when plural images with different colors are formed superposingly on the same image receiving medium, and an image forming apparatus for carrying out such a method (U.S. Ser. No. 679,903 filed on Apr. 3, 1991 corresponding to Japanese Patent Application No. 2-88168).